Researchers,
acting as "genetic paleontologists," recently discovered that the X and
Y chromosomes of cats and humans are remarkably alike, despite the fact
that the two species haven't shared a common ancestor for about 90 million
yearsaround the same time the human line diverged from goats, sheep,
and cows. This similarity doesn't govern sexual behavior, as sexuality
is not determined by genes found on X and Y chromosomes. But it may help
scientists better understand male infertility, human genetics, and may
even help preserve endangered cat species.

William Murphy, Stephen O'Brien, and their colleagues at the U.S. National
Cancer Institute (NCI) examined 25 unique DNA sequences on the X chromosomes
of cats, mice and human beings, and looked at eight genes on the Y chromosomewith
startling results.

The order of these genes on the sex chromosomes of cats more closely
resembles that of humans than of any other mammal studied so far, the
team reports in a recent issue of Genome Research. Genes on X chromosomes
of cats and people are lined up in the same way. This long-term chromosome
stability is rare, showing no evidence of evolutionary rearrangements,
says O'Brien. "It's like there was some master template that maintained
the order of these genes on the X chromosome from the distant ancestors
of these two mammalian orders."

Most X chromosomes are home to a variety of genes, including what Murphy
calls "essential housekeeping genes," genes involved in basic cellular
functions such as metabolism. This is why all mammals have the same genes
on X chromosomes, but in various arrangements. Studies have shown that
the same genes in mice, cows, goats and rats are arranged in different
sequences.

The researchers also isolated a cluster of three genes on the Y, or male,
chromosome in cats, mice and humans that have maintained identical order
and spacing over the millennia. This conservation appears to be important
for sperm production. When regions of the Y chromosome harboring those
genes in cats are deleted, Murphy says, the males are infertilea
fact which may make the cat a useful model to study infertility. Of all
seven genes the group studied on the Y chromosome, only one is in a different
order in cats than it is in humans.

About half of the current X chromosome is like a fossil, having remained
the same through the evolution of all mammals, even the relatively primitive
marsupials. Genes on the remainder of the X in placental mammals, on the
other hand, were more recently acquired from other chromosomes. Some researchers
suggest that some genes cycled from regular chromosomes to the X chromosome
and then to the Y chromosome. Once on the Y, they acquired a male-specific
function or were lost.

Because the sex chromosomes of cats bear such a close similarity to those
of humans, cats may come into their own as valuable surrogates for studying
a range of human maladies. O'Brien says almost 200 human hereditary diseases
also manifest in cats, including diabetes, Tay Sachs disease and hemophilia.
Furthermore, certain viruses behave in cats much as they do in humans.
Feline leukemia and FIV, the Feline Immunodeficiency Virus (which is very
similar to HIV), rank high on this list.

The house cat belongs to a family that has rich diversity in nature with
37 species. Males of certain of its wild relatives, notably the Florida
panther, cheetah and clouded leopard, suffer from poor reproductive ability.
As their populations dwindled, these species underwent evolutionary bottlenecks,
leading to inbreeding. As a result, they have low sperm counts with a
high degree of structural abnormalities in the sperm. While this is bad
news for the species, it may make these cats good candidates for infertility
studies. Such research might benefit not only humans, but the cats as
well. "Every species except the house cat is threatened or endangered,"
says O'Brien. Research on human infertility might turn up ways to help
the endangered wild cats.

O'Brien hopes cats will get their own genome project. "I believe we will
add the cat to the list. It, too, may be finished within the next decade,
depending on funding" he says. Tracing our genetic ancestry will not only
unearth details about our evolution, but may help us find the cures to
our most debilitating human diseases.